1. Field of the Invention
This application is to apply for the priority insistence by the assumption of laid open patent JP 2001-251,407 and JP 2001-251423 basic.
The present invention relates to a light emitting device, particularly to a device that can emit uniform light with less color heterogeneity, for example, white light of high output power.
2. Description of the Related Art
Portable electronic devices, sign lamps, indication lamps and other devices are increasingly employing semiconductor light emitting elements, for example, light emitting diode (LED), that have high luminance and low power consumption, as the light sources. When such a semiconductor light emitting element is used to constitute a light emitting device, light emitted by the semiconductor light emitting element that is a point light source must be converted into planar emission of light.
Prior art of achieving planar light emission includes such techniques as a film that contains a light diffusion agent and is mounted in front of a semiconductor light emitting element chip, a lens that is mounted in front of a semiconductor light emitting element chip to control the directivity of light, and an array of semiconductor light emitting element chips arranged with a high density so as to provide quasi planar light emission.
However, the film including the light diffusion agent results in a low light transmissivity due to the light diffusion agent, while the technique using the lens or high density array results in higher manufacturing cost.
On the other hand, it has been proposed to form a reflector around a semiconductor light emitting element so as to reflect a part of the light from the semiconductor light emitting element forward, thereby achieving planar light emission.
Recently, the emission of white light have been increasingly demanded. Devices for emitting white or near white light are often made by using a semiconductor light emitting element that emits blue light and a YAG series fluorescent substance (yttrium aluminum garnet) dispersed in a sealing resin, so that a part of the light from the semiconductor light emitting element is absorbed by the fluorescent substance that in turn emits yellow light through wavelength conversion, and the blue light and the yellow light are blended so as to produce white light.
With this method of mixing the YAG series fluorescent substance in the sealing resin, uniform white light with less color heterogeneity can be produced, although light transmissivity through the sealing resin is lowered by the fluorescent substance, thus making it difficult to achieve a satisfactory optical output power.
In order to overcome such drawbacks, many variations of method of coating a reflector with a YAG series fluorescent substance have been proposed (see Japanese Unexamined Patent Publications (Kokai) No. 10-112557 (1998), No. 10-319877 (1998) (div. No. 2000-81847), No. 11-87778 (1999) (div. No. 2000-82849), No. 11-274572 (1999)).
However, with the structures that have been disclosed in the above patent publications so far, light emitted from the side faces of the semiconductor light emitting element has not been fully utilized.
Also with the structures disclosed in the above patent publications, there is such a problem that light is not fully diffused in a cavity that houses the semiconductor light emitting element, thus resulting in the emission of white light having color heterogeneity with the semiconductor light emitting element emitting bluish light and the reflector emitting yellowish light.
An object of the present invention is to provide a light emitting device that can emit uniform white light with less color heterogeneity and high output power.
The light emitting device of the invention comprises a semiconductor light emitting element emitting at least part of light from side faces thereof; and a fluorescent member provided at a distance from the semiconductor light emitting element; the fluorescent member being capable of emitting light by wavelength-conversion due to absorption of part of light emitted from the semiconductor light emitting element and including at least yttrium aluminum garnet fluorescent substance as a component having at least one element selected from the group consisting of Y, Lu, Sc, La, Gd and Sm, at least one element selected from the group consisting of Al, Ga and In, at least Ce and at least one element selected from the group consisting of Pr, Sm, Cu, Ag, Au, Fe, Cr, Nd, Dy, Ni, Ti, Tb and Eu; wherein the fluorescent member has a first portion at a first distance from the semiconductor light emitting element and a second portion at a second distance farther than the first distance from the semiconductor light emitting element and the first portion has a higher efficiency of absorption of light emitted from the semiconductor light emitting element than that of the second portion.
Using the above fluorescent substance allows the color of light to obtain high color rendering properties.
In addition, designing the higher efficiency of absorption of light emitted from the semiconductor light emitting element at each portion of the fluorescent member as the portion being at the shorter distance from the semiconductor light emitting element, allows the light emitted from the side faces of the semiconductor light emitting element to be utilized more efficiently. Thus, the ratio in quantity of light wavelength-converted by the fluorescent substance out of light irradiated from the light emitting element becomes substantially uniform over the whole fluorescent member, thereby light of uniform color with high color rendering properties and less color heterogeneity can be emitted.
The light emitting device of the invention also comprises a semiconductor light emitting element emitting at least part of light from side faces thereof; and a fluorescent member provided at a distance from the semiconductor light emitting element; the fluorescent member being capable of emitting light by wavelength-conversion due to absorption of part of light emitted from the semiconductor light emitting element; wherein the fluorescent member has a first portion at a first distance from the semiconductor light emitting element and a second portion at a second distance farther than the first distance from the semiconductor light emitting element and the first portion has a higher efficiency of absorption of light emitted from the semiconductor light emitting element than that of the second portion.
With this constitution, designing the higher efficiency of absorption of light emitted from the semiconductor light emitting element at each portion of the fluorescent member as the portion being at the shorter distance from the semiconductor light emitting element, allows the light emitted from the side faces of the semiconductor light emitting element to be utilized more efficiently. Thus, the ratio in quantity of light wavelength-converted by the fluorescent substance out of light irradiated from the light emitting element becomes substantially uniform over the whole fluorescent member, thereby light of uniform color with high color rendering properties and less color heterogeneity can be emitted.
According to the invention, it is preferable that the fluorescent member is formed of a transparent resin material with a fluorescent substance dispersed therein, and said efficiency of absorption is changed continuously or stepwise correspondingly to the absolute quantity of the fluorescent substance.
The efficiency of absorption at each portion of the fluorescent member is adjustable continuously or stepwise by changing the absolute quantity of the fluorescent substance continuously or stepwise.
According to the invention, it is preferable that the first portion has a higher density of the fluorescent substance in the fluorescent member than that of the second portion.
The efficiency of absorption at each portion of the fluorescent member is also adjustable continuously or stepwise by changing the density of the fluorescent substance continuously or stepwise.
According to the invention, it is preferable that the fluorescent member includes multiple layers stacked, each layer including different kind of fluorescent substance and said efficiency of absorption in each layer is changed correspondingly to the light conversion efficiency of each fluorescent substance.
The efficiency of absorption at each portion of the fluorescent member is also adjustable continuously or stepwise by stacking multiple layers, each of layer including different kind of fluorescent substance and changing the light conversion efficiency of each fluorescent substance.
According to the invention, it is preferable that the fluorescent member contains a fluorescent substance and a light-diffusing agent as components, and said efficiency of absorption is changed correspondingly to the density of the light-diffusing agent.
The efficiency of absorption at each portion of the fluorescent member is also adjustable continuously or stepwise by containing a light-diffusing agent and changing the density of the light-diffusing agent.
According to the invention, it is preferable that the first portion has a higher density of the light-diffusing agent in the fluorescent member than that of the second portion.
Designing the density of the light-diffusing agent at each portion of the fluorescent member as the portion being at the shorter distance from the semiconductor light emitting element, allows the light emitted from the side faces of the semiconductor light emitting element to be utilized more efficiently. Thus, the ratio in quantity of light wavelength-converted by the fluorescent substance out of light irradiated from the light emitting element becomes substantially uniform over the whole fluorescent member, thereby light of uniform color with less color heterogeneity can be emitted.
According to the invention, it is preferable that the fluorescent member is provided near the side faces of the semiconductor light emitting element and has a flat or curved surface that reflects light emitted from the semiconductor light emitting element.
This constitution allows the light emitted from the side faces of the semiconductor light emitting element to reach the fluorescent substance to be utilized for fluorescent substance more efficiently. In addition, the fluorescent member has a flat or curved surface that reflects light emitted from the semiconductor light emitting element, so that the light emitted from the semiconductor light emitting element and the light emitted from the fluorescent substance can be blended efficiently and light of uniform color with less color heterogeneity can be emitted.
According to the invention, it is preferable that the device generate a light mixed with both the visible light emitted from the semiconductor light emitting element and the visible light emitted from the fluorescent substance.
Thus, mixing with both the visible light emitted from the semiconductor light emitting element and the visible light emitted from the fluorescent substance allows the utility efficiency of light to upgrade.
According to the invention, the mixed light is preferably white light, thereby the light emitting device can be used as a back light source for any apparatuses such as illuminating apparatus and signal light.
The light emitting device of the invention also comprises electrodes provided on a base surface; a semiconductor light emitting element that is mounted on the base surface at a distance from the electrodes and emits part of light from side faces thereof; a connecting member that electrically connects the semiconductor light emitting element and the electrodes; and a fluorescent member formed of a transparent resin material with a fluorescent substance dispersed therein, the fluorescent substance capable of emitting light by excitation of the light emitted from the semiconductor light emitting element, the fluorescent member being provided to surround the semiconductor light emitting element; wherein the fluorescent member has a curved surface that reflects the light emitted from the side faces of the semiconductor light emitting element away from the base surface.
One feature of the invention is that the fluorescent member has a curved surface of concave arc that extends from the base surface obliquely upward to the outside at least in the lower portion of the fluorescent member, while the fluorescent member includes a curved surface that reflects the light emitted from the side faces of the semiconductor light emitting element away from the base surface, and the fluorescent member includes a fluorescent member dispersed therein, the fluorescent substance capable of emitting light by excitation of the light emitted from the semiconductor light emitting element.
With this constitution, not only the light emitted obliquely forward from the semiconductor light emitting element but also most of the light emitted from the side faces thereof can reach the fluorescent substance included in the fluorescent member so as to be effectively used by the fluorescent substance to emit light. As a result, light transmissivity can be made higher and a higher optical output power can be achieved than with the conventional structure wherein the sealing resin includes the fluorescent substance dispersed therein.
Also because the lower portion of the fluorescent member has at least the inner surface of the lower portion formed in a curved surface of concave arc, part of the light emitted from the semiconductor light emitting element and most of the light emitted from the side faces are more likely to be diffused in the fluorescent member than in the flat reflector structure, so that the light emitted from the semiconductor light emitting element and the light emitted from the fluorescent substance can be blended efficiently and light of uniform color with less color heterogeneity can be emitted.
While the fluorescent member may have at least the inner surface of the lower portion in the direction of height formed in a curved surface of concave arc, part of the light emitted from the semiconductor light emitting element and most of the light emitted from the side faces can be utilized more efficiently so that optical output power can be increased greatly by forming the fluorescent member over the entire height in a curved surface of concave arc.
Moreover, since the inner edge at the lower end of the fluorescent member is set at the height of the base surface or that of the vicinity thereof, most of the light emitted from the side faces of the semiconductor light emitting element can reach the curved surface. But it is preferable to keep a space between the inner edge at the lower end of the fluorescent member and the side faces of the semiconductor light emitting element which causes all of the light emitted from the side faces of the semiconductor light emitting element to surely reach the curved surface of the fluorescent member, thereby increasing the optical output power more reliably.
According to the invention, the inner surface of the fluorescent member is preferably formed so as to substantially constitute a part of paraboloid of revolution, ellipsoid of revolution or hyperboloid of revolution.
With this constitution, the light emitted from the semiconductor light emitting element can be collected efficiently, while the light emitted from the semiconductor light emitting element and the light emitted from the fluorescent substance can be blended efficiently and the efficiency of utilizing the light is improved.
The light emitting device of the invention also comprises electrodes provided on a base surface; a semiconductor light emitting element that is mounted on the base surface at a distance from the electrodes and emits part of light from side faces thereof; a connecting member that electrically connects the semiconductor light emitting element and the electrodes; and a fluorescent member formed of a transparent resin material with a fluorescent substance dispersed therein, the fluorescent substance capable of emitting light by excitation of the light emitted from the semiconductor light emitting element, the fluorescent member being provided to surround the semiconductor light emitting element; wherein density of the fluorescent substance included in the fluorescent member is changed continuously or stepwise with the height from the base surface.
Another feature of the invention is that the fluorescent member is formed of the transparent resin material and disposed to surround the semiconductor light emitting element, and the fluorescent substance dispersed in the fluorescent member can emit light by excitation of the light emitted from the semiconductor light emitting element, with the density of the fluorescent substance being changed continuously or stepwise with the height from the base surface.
With this constitution, not only the light emitted obliquely forward from the semiconductor light emitting element but also most of the light emitted from the side faces thereof can reach the fluorescent substance included in the fluorescent member so as to be effectively used by the fluorescent substance to emit light. As a result, light transmissivity can be made higher and a higher optical output power can be achieved than with the conventional structure wherein the sealing resin includes the fluorescent substance dispersed therein.
According to the present invention, density of the fluorescent substance in the fluorescent member is preferably made higher as the height from the base surface decreases.
By dispersing the fluorescent substance in the fluorescent member so that the density on the base surface side higher than the density on the surface side, part of the light emitted from the semiconductor light emitting element and most of the light emitted from the side faces are more likely to be diffused than in the conventional structure that is coated with a fluorescent substance. Thus the light emitted from the semiconductor light emitting element and the light emitted from the fluorescent substance can be efficiently blended so as to produce a desired color, and light of uniform color with less color heterogeneity can be produced.
While it suffices to make the density of fluorescent substance higher on the base surface side than on the surface side, such a distribution as the density of fluorescent substance gradually decreases from base surface side to the surface side makes it possible to more efficiently use part of the light emitted from the semiconductor light emitting element and most of the light emitted from the side faces, thereby significantly increasing the optical output power.
According to the invention, the transparent resin material of the fluorescent member is preferably an epoxy resin, silicone resin, amorphous polyamide resin or fluorocarbon. With such a constitution, reliable emission of light by the fluorescent substance dispersed in the material can be achieved.
Also according to the invention, the fluorescent substance in the fluorescent member is preferably yttrium aluminum garnet (YAG) series.
In the present invention various fluorescent substances such as inorganic or organic fluorescent substance may be included in the fluorescent member. For example, an inorganic fluorescent substance containing a rare earth element, specifically, a fluorescent substance of garnet structure having at least one element selected from the group consisting of Y, Lu, Sc, La, Gd and Sm, and at least one element selected from the group consisting of Al, Ga and In is available. More specifically, a fluorescent substance of yttrium aluminum oxide activated by Ce is preferable, if required, Pr, Sm, Cu, Ag, Au, Fe, Cr, Nd, Dy, Ni, Ti, Tb and/or Eu may be contained with Ce. Particularly, a fluorescent substance containing both Ce and Pr can emit the color of light with high color rendering properties.
When a light emitting diode having light emitting layer made of gallium nitride semiconductor that has a high band gap energy is used for the semiconductor light emitting element, for example, a fluorescent substance activated with divalent europium having chemical composition of (M1xe2x88x92pxe2x88x92qEupQq)O.n(Al1xe2x88x92mBm)2O3 is preferably used as described in Japanese Unexamined Patent Publication (Kokai) No. 10-112557 filed by the present applicant. The proportions of the elements are 0.0001xe2x89xa6pxe2x89xa60.5, 0.0001xe2x89xa6qxe2x89xa60.5, 0.5xe2x89xa6nxe2x89xa610, 0xe2x89xa6mxe2x89xa60.5, and 0.0002xe2x89xa6p+qxe2x89xa60.75. In the chemical formula described above, M represents at least one element selected from the group of divalent metals consisting of Mg, Ca, Sr, Ba and Zn, Q represents a co-activating agent that is at least one element selected from the group consisting of Mn, Zr, Nb, Pr, Nd, Gd, Th, Dy, Er, Tm, Yb and Lu.
When the fluorescent substance having the composition described above is used, the fluorescent substance will not deteriorate even after emitting light that has high energy in the visible region for a long period of time, thus ensuring a desired luminance and after glow over a long period of time.
Also according to the invention, the fluorescent substance included in the fluorescent member is preferably a nitride fluorescent substance that includes nitrogen at least in the fundamental elements.
An example of nitride fluorescent substance that can be used is one that includes fundamental constituent elements in composition of LxMyN(2X/3+4Y/3):Z (L represents at least one element selected from the group of divalent elements consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg, M represents at least one element selected from IV group elements consisting of C, Si, Ge, Sn, Ti, Zr, Hf, and Z represents at least one element selected from a group of elements consisting of Eu, Cr, Mn, Pb, Sb, Ce, Tb, Pr, Sm, Tm, Ho, Er, Yb, Nd) and at least one additional element selected from the group consisting of Mg, Sr, Ba, Zn, B, Al, Cu, Mn, Cr, O and Fe. Such a nitride fluorescent substance allows it to regulate the particle size and improve the luminance of light emission because of the additional elements included therein. B, Mg, Cr, Ni and Al have the effect of suppressing the after glow.
The fluorescent substance may be used in a single kind or mixture of two or more kinds. When two or more kinds of fluorescent substance are used, it is preferable that they emit light of colors that are complementary to each other.
In one embodiment, the light emitting device comprises a semiconductor light emitting element that emits first light, a first fluorescent substance that is excited by a part of the first light so as to emit second light, and a second fluorescent substance that is excited by a part of the first light so as to emit third light. For example, with a constitution employing a semiconductor light emitting element that emits light of a bluish color (dominant wavelength 455 nm) and a fluorescent member including Y3(Al0.8Ga0.2)5O12:Ce that is excited by light of wavelength (excitation wavelength 440 nm) shorter than the dominant wavelength of the semiconductor light emitting element so as to emit greenish light (dominant wavelength 530 nm) as the first fluorescent substance, and (Sr0.679Ca0.291Eu0.03)2Si5N8 that has an excitation wavelength substantially the same as that of the first fluorescent substance and emits reddish light (dominant wavelength 650 nm) as the second fluorescent substance, a light emitting device that emits warm white light with high color rendering properties by blending the light of the three wavelengths (bluish, greenish and reddish colors).
Incidentally, the term xe2x80x9cwhite colorxe2x80x9d in the present specification means a color classified in the white regions of the xe2x80x9cAppendix Diagram 1, General Chromaticity Regions corresponding to Name of System Color, defined by JIS Z8110-1995xe2x80x9d.
A light emitting device that emits warm white light can also be made by combining a semiconductor light emitting element that emits light in ultraviolet region and a plurality of fluorescent substances that absorb the ultraviolet ray and emit visible light, and blending the light emitted by the different fluorescent substances.
In case a plurality of fluorescent substances are used to form multiple layers of color conversion wherein multiple color conversion thin films, each including different kind of fluorescent substance, are stacked, it is preferable to set the ultraviolet ray transmissivity of an upper layer higher than that of a lower layer that is on the substrate side, while giving consideration to the ultraviolet ray transmissivity of each fluorescent substance.
Median particle size of the fluorescent substance included in an upper layer is preferably smaller than that of a lower layer that is on the substrate side. With this arrangement, the fluorescent substance included in the topmost layer can be efficiently irradiated with ultraviolet ray, and the ultraviolet ray can be prevented from leaking to the outside of the device.
When the red light emitting fluorescent substance, the blue light emitting fluorescent substance and the green light emitting fluorescent substance described above are used, for example, it is preferable to stack the red light emitting fluorescent substance on the substrate, then the green light emitting fluorescent substance and in turn the blue light emitting fluorescent substance thereon. Median particle size of the fluorescent substance is preferably largest in the red light emitting fluorescent substance, smaller in the green light emitting fluorescent substance and smallest in the blue light emitting fluorescent substance.
According to the invention, it is preferable that the fluorescent member includes a pigment having a body color similar to the color of light emitted from the fluorescent substance. For example, color of emitted light can be made clearer by using a pigment that can shut off light in a region of the emission spectrum of the semiconductor light emitting element that is not the complementary color of the light emitted from the light emitting device or light that does not contribute to the excitation of the fluorescent substance. In this case, the effect can be made higher by setting the body color of the pigment similar to that of light emitted by the fluorescent substance.
In order to have the fluorescent substance dispersed in the fluorescent member, it is preferable to use such a synthetic resin having viscosity that may harden while keeping the fluorescent substance from precipitating in the resin, so that all of the fluorescent substance does not precipitate in the lower portion of the reflector.
The fluorescent member is preferably formed of a transparent synthetic resin material that has viscosity in a range from 2,500 mPaxc2x7s to 20,000 mPaxc2x7s, preferably from 3,000 mPaxc2x7s to 10,000 mPaxc2x7s, more preferably from 4,000 mPaxc2x7s to 8,000 mPaxc2x7s.
While quantity of the fluorescent substance may be determined according to the wavelength and intensity of light emitted from the semiconductor light emitting element and other condition, it is preferable to mix 40 to 300 parts by weight, preferably 40 to 200 parts by weight, more preferably 40 to 100 parts by weight of the fluorescent substance with 100 parts by weight of the synthetic resin material.
While efficiency of the fluorescent substance to emit light by excitation of the light emitted from the semiconductor light emitting element becomes higher as the particle size of the fluorescent substance is larger, larger fluorescent substance particles are more likely to precipitate in the lower portion of the fluorescent member. Therefore, mean particle size of the fluorescent substance included in the fluorescent member is preferably in a range from 6 to 25 xcexcm as measured by Fischer""s method.
While the curved surface of the fluorescent member may be formed by chemical process such as etching, it may also be formed spontaneously by potting a transparent resin material including the fluorescent substance mixed therein, which leads to simplified manufacturing process and lower cost.
According to the invention, it is preferable to seal the semiconductor light emitting element with a sealing resin. There is not particular restriction on the sealing resin as long as it is transparent and, for example, epoxy resin, silicone resin, amorphous polyamide resin or fluorocarbon may be used.
When a resin layer that contains a light-diffusing agent is provided above the semiconductor light emitting element by coating or other process, color heterogeneity can be prevented more reliably and light of uniform color can be emitted. For the light-diffusing agent, one or more selected from among barium titanate, titanium oxide, aluminum oxide, silicon oxide, calcium carbonate, white carbon, talc and magnesium carbonate may be used.
When a light collecting member such as single lens or Fresnel lens is provided above the semiconductor light emitting element, desired directivity of light can be easily achieved and the light emitting device can be used as a back light source for any apparatuses such as illuminating apparatus and signal light. Use of a Fresnel lens as the light collecting member enables it to build a thin light emitting device.